The present invention relates to attachments for stringed instruments and pertains particularly to an improved capo for fretted stringed instruments. In use the capo enables rapid key changes. It is frequently desirable to raise the pitch of the strings of fretted stringed musical instruments which is typically accomplished by means of a device called a capo. A capo comprises a bar which is clamped across the strings for pulling the strings to the frets which are embedded in the fingerboard or neck of the instruments. This shortens the effective vibrating length of the strings, thereby raising the pitch thereof. The capo is moved to various fret locations along the finger board for selectively raising the pitch of the instrument. Relatively large forces are required to simultaneously clamp all strings with a straight rigid bar due to the adverse profile imposed by the tops of strings of different diameter in contact with frets which may vary in profile across the width of the fingerboard. Several devices have been devised to increase the clamping force to effectively clamp the strings to the frets. These prior art devices, however, are unwieldy, require unusual manual dexterity to use, and must be removed or loosened to change position along the finger board and then re-tightened. When not in use, they are temporarily attached to the instrument away from the strings or removed from the instrument. In my prior capo patent I disclosed an improved capo having a flexible bar assembly with a full wear sleeve that progressively applies pressure to the strings by force of a spring in a low friction spring sheath to bias them to add force against the frets. Problems with the design of my prior capo are; loss of force applied by the bar to the strings near the center of a fingerboard having a flat profile during flexure of the bar assembly, excessive effect of wear sleeve stiffness on flexure of the bar, difficulty to stretch the spring during assembly of the capo to the instrument and potentially insufficient wear resistance of the spring sheath. I have discovered further improvements in component configuration that resolve these problems and are incorporated herein. It is desirable that the capo be capable of uniform application of force to all of the strings against the frets, and be simple and easy to move to selected locations along the neck of the instrument.